Dimethyl phosphonate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

24.8 µg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

248 µg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

2.48 µg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1 000 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

54.01 µg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

5.4 µg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

4.56 µg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Conclusion on classification

Ecotoxicological classification of dimethyl phosphonate cannot be determined unequivocally on the basis of data available and may need further discussion and investigations, especially on the hydrolysis product monomethyl phosphonate.

An important characteristic of dimethyl phosphonate is that it is hydrolysed in water with a half-life strongly depending on the pH. According to the “Guidance on the Application of the CLP criteria” (ECHA, 2009) hydrolysis half-life must be considered between pH values of 4 to 9. The longest hydrolysis half-life of dimethyl phosphonate was found to be 470 h (19.6 d) at pH 4 (Bayer AG, 2002) which indicates that the substance can remain in water for more than 12 h and therefore dimethyl phosphonate itself and not its hydrolysis products should be considered for classification. Nevertheless dimethyl phosphonate is hydrolysed significant faster at pH 7 and pH 9 with half-lifes of 3 h and less than 1 min, respectively.

Tests on biodegradation, which are supposed to be conducted at neutral pH, revealed dimethyl phosphonate itself to disappear up to 86% during 28 days based on analysis of dimethyl phosphonate itself (MITI, 1992). Dimethyl phosphonate elimination is based primarily on hydrolysis. Measurements on DOC, BOD and TOC showed that only 50% of the initial carbon attributed to dimethyl phosphonate was mineralised during the biodegradation tests while half of the initial carbon remained in solution (Bayer AG, 1992; MITI, 1992). Since dimethyl phosphonate disappeared up to 86% and the hydrolysis product methanol is readily biodegradable, most of the remaining carbon is likely to be attributed to the hydrolysis product monomethyl phosphonate. According to the “Guidance on the Application of the CLP criteria” (ECHA, 2009) hydrolysis can only be used as an argument for ready biodegradation “when the longest half-life t1/2 determined within the pH range 4-9 is shorter than 16 days” and “only when it can be satisfactorily demonstrated that the hydrolysis products formed do not fulfil the criteria for classification as hazardous for the aquatic environment”. Since both criteria are not fulfilled and measured BOD- and DOC values only reached 50% dimethyl phosphonate is considered not to be readily biodegradable.

Based on calculated very low log Kow values dimethyl phosphonate and its hydrolysis products are not expected to accumulate in biota.

In aquatic toxicity tests a toxic effect on the crustacean Daphnia magna was found (Bayer AG, 2003). After 48 h a nominal EC50 of 24.8 mg/L was determined. Due to hydrolysis dimethyl phosphonate concentrations dropped down below its detection limit during the test, whereas significant monomethyl phosphonate concentrations (22 – 35% w/w of initial dimethyl phosphonate) were present throughout the whole exposure period. During hydrolysis of dimethyl phosphonate the hydrolysis products formed are methanol, phosphonic acid and monomethyl phosphonate. Methanol and phosphonic acid are known not to be toxic to the environment. Information is available on both substances. In contrast, no data is available on toxicity of monomethyl phosphonate. Since it is not obvious whether the effect on Daphnia magna is caused by dimethyl phosphonate or monomethyl phosphonate, a toxic effect of dimethyl phosphonate cannot be excluded. For security reasons it was decided to classify dimethyl phosphonate with R52/53 according to 67/548/EEC and with aquatic chronic category 3, H412 according to CLP. A classification of dimethyl phosphonate could be avoided if data would be available on monomethyl phosphonate which indicate a toxic effect of this substance on Daphnia magna or if long-term studies of dimethyl phosphonate on Daphnia magna would be available which indicate a NOEC > 1 mg/L.